As the level of semiconductor circuit integration continues to increase and the complexity of these circuits advances, circuit reliability is becoming more dependent upon an increasing number of factors. For example, aggregate current leakage and increases in circuit speed generate greater thermal loads, while the use of low-k dielectric materials and air gaps in the dielectric materials adversely impacts dissipation of this heat. Smaller device sizes, increased packing density, advanced packages that trap heat in upper metallization, and shrinking line widths contribute to performance and reliability concerns. With oxide scaling there is an increased ratio of aggregate off current (Ioff) to active device switching current, (Ion). There are also an increased circuit complexity and a greater variability in conditions under which devices are operated.
Due to the foregoing factors it is becoming ever more difficult to use conventional techniques like accelerated testing to assess the long-term stability of integrated circuit devices. Changes in local device operating temperature, isolated thermomigration, consequent materials transformations, and the rate of electromigration can be underestimated, resulting in premature and unexpected parametric and functional device failure.